1. Field of Application
The present invention relates to an exposure control apparatus, for exposure control of an electronic camera which captures successive images of a region ahead of a vehicle in which the camera is installed.
2. Description of Related Art
In recent years, vehicle-installed electronic cameras (in general, digital video cameras, referred to in the following simply as cameras) have come into use for capturing images of a region located ahead of the vehicle, with technology having been developed whereby the captured images (sets of digital data expressing respective captured images) are processed for such functions as recognition of white lines formed on the road surface or detection of objects such as a preceding vehicle. The processing results can be used to generate warning indications to a vehicle driver, control driving of the vehicle, etc.
With such technology, it is important that the exposure of the camera be appropriately controlled in accordance with brightness of the scene which is being captured by the camera, in order to maximize the reliability of recognizing white lines on the road surface or solid objects such as preceding vehicles, etc., based on the images captured by the camera.
A vehicle having such a camera and processing apparatus installed therein, which are being described, is referred to in the following as the “local vehicle”.
As described for example in Japanese patent first publication No. 6-253208 (designated as reference document 1 herein), a method of using such camera images for recognition of white lines on the road surface has been proposed whereby two laterally extending sections are selected within each image. A first one of these sections is positioned to contain a part of the road surface that is currently close to the local vehicle, and the data obtained from the first section, in each of successive captured images, are utilized for recognition of white lines on the road surface. The second section is positioned to contain a part of the road surface that is farther ahead of the local vehicle (i.e., is in an upper part of each captured image). Hence the second section contains a region which will be subjected to recognition processing at a future time point, determined by the speed at which the local vehicle is travelling. Designating the average brightness levels of the first and second sections as b0 and b1 respectively, the difference between these is obtained for each of successive captured images. If the difference is found to exceed a predetermined threshold value, then the camera exposure which will be applied in capturing the next image is adjusted based on the brightness value b1 (i.e., by changing the camera shutter speed, etc).
With the above method of reference document 1, if for example the road surface ahead of the vehicle changes between a brightly sunlit condition and a shade condition, the camera exposure can be appropriately controlled for each of successive captured images, i.e., such as to prevent the abrupt change in scene brightness from affecting the reliability of white line detection.
However in an actual road environment, the brightness of the road surface will not generally change between a sunlit condition and a shade condition (with the change extending uniformly across the road surface) in such a simple manner. Instead, the changes can take various forms. For that reason, it is difficult to reliably control the camera exposure by such a method under actual operating conditions.
It has also been proposed, for example in Japanese patent first publication No. 2005-148308 (designated as reference document 2 herein) to use an exposure control apparatus whereby the brightness of the road surface ahead of a local vehicle is measured for use in camera exposure control, while excluding the effects of white lines (traffic lane markers) formed on the road surface. A video camera on the vehicle obtains successive captured images of A region directly ahead of the vehicle, and a plurality of brightness measurement regions are determined, within each image, which are designed to exclude these white lines. The exposure of the vehicle-mounted camera is controlled based on measured brightness values of these brightness measurement regions. However it is difficult to form and locate these brightness measurement regions such as to ensure that the white lines will be consistently excluded from them. Hence it has been difficult, in practice, to use such brightness measurement regions for performing reliable brightness measurement based on brightness values in these regions.
With a vehicle-installed exposure control apparatus, it is necessary to achieve a high speed of processing. This can be increased by applying processing to only a part of the picture elements which constitute the brightness measurement region, rather than deriving brightness values based upon all of these picture elements. However if this is done, the problem arises that dispersed large-amplitude variations may occur in the brightness values of some parts of the brightness measurement region. This is especially true for part of the brightness measurement region which correspond to external regions that are relatively distant.
One reason for this will be described referring to the example of FIG. 17, which relates to an exposure control apparatus in which “thinning-out” is applied whereby some picture lines (rows of picture elements) 52 are excluded from a brightness measurement region in a captured image 48, i.e., brightness values of these picture lines are omitted from being used in brightness measurement processing. The upper diagram in FIG. 17 conceptually shows a captured image in which a preceding vehicle 50 appears, with light emitted from the tail lamps 51 of the vehicle 50 appearing in the image at positions corresponding to one of the excluded picture lines 52, as indicated by the arrow 53. When pitching motion of one or both of the vehicles occurs, then in a subsequent image as illustrated by the lower diagram, the light from the tail lamps 51 may correspond to positions other than the excluded picture lines 52, as indicated by arrow 54. Hence, a large increase in the measured brightness value obtained for this image (e.g., average brightness of the brightness measurement region) will occur. Such fluctuations in measured brightness values make it difficult to apply effective exposure control.
The greater the distance of a light source such as a preceding vehicle tail lamp, etc., the smaller will be the apparent size of the light source within a captured image. Hence, such fluctuations in brightness will increase in accordance with increasing distance of external regions represented in an image.